Microscopic Mechanism Research On Energy And Mass Transport Between Salt Aqueous Solution And Humid Air

Halide salt aqueous solutions are employed to remove moisture in air because of hygroscopicity,which is driven by the vapor pressure difference.Liquid desiccants could also be regenerated by taking advantage of solar energy or waste heat.So,liquid desiccant dehumidification shows significant energy-saving potential and is widely applied in air conditioning.Heat and mass transfer between aqueous solutions and air are vital to dehumidification or regeneration.Although extensive experiments performed could reveal the heat and mass transfer characteristics under different dynamics parameters,the effects of thermodynamic parameters are still ambiguous and developed empirical correlations of mass transfer coefficients are non-universal.This poses the difficulty for equipment development and performance evaluation.In addition,enhancement measures of mass transfer are limited and mechanism researches are particularly scarce.Thus,molecular dynamics is combined with experiments to disclose the mechanism of energy and mass transport between aqueous solutions and air in this paper,which sets a foundation for developing novel methods of mass transfer intensification and building uniform correlations of mass transfer coefficients.The detailed researches and results are as follows.Firstly,the suitable all-atom force fields were selected to describe particle interactions in aqueous solutions,which ensured the accuracy of molecular dynamics simulations.On the basis of that,difference between the interface layer and liquid bulk were discussed.Particle number density,radial distribution function,hydrogen bond distribution,water orientation and orientation of water around ions were analyzed to reflect configurations and main interactions in liquid-vapor systems.Effects of solution temperature and concentration on the water diffusivity and interface layer thickness were studied to obtain the variation of interfacial resistance.Results show that with an increase in the solution temperature or a decrease in the solution concentration,the particle distribution along the interface normal direction is more even and the particle number in the interface layer rises.The appearance of ions depresses the formation of hydrogen bonds.As a result,hydrogen bonds are not dominant in aqueous solutions.Besides,the diploe moment direction of water molecules is random for the liquid bulk but points to the air side slightly for the interface layer.Furthermore,the number of water molecules around Li~+is more than that around halide ions at close range,which means the hydration of Li~+is stronger.The water diffusion coefficients in the interface layer are anisotropic with the normal component much smaller than the tangential component,but are larger than in the liquid bulk.As the solution concentration goes up,the ratio of the interface thickness to the average interface diffusion coefficient rises,which indicates the augment of the mass transfer resistance in the interface layer.Secondly,in order to investigate the molecular transport behavior during dehumidification/regeneration,non-equilibrium molecular dynamics simulations were performed in liquid-vapor systems involving lithium bromide aqueous solutions and air.Effects of solution temperature,solution concentration,density of dry air and density of water vapor in air were explored.Moreover,contributions of molecular behaviors to the energy and mass transport were analyzed.It is found that absorption,reflection and displacement may occur when the water molecules in the air move to the liquid bulk.And reflection includes interface reflection and gas space reflection.Similarly,release,reflection and displacement may occur when the water molecules in the interface layer move to the air side.So,both dehumidification and regeneration are the comprehensive results of aforementioned four behaviors.The main impact of larger water vapor densities is increasing absorption while the main impact of larger dry air densities is increasing reflection.If the densities of all components in air enlarge simultaneously,net water absorption will become larger due to the higher increase proportion of absorption.Although water interface reflection has no contribution to the net mass transport but it enhances the net energy transport.In addition,both absorption and release behaviors show direct impact on net mass and energy transport.The molecular transport behaviors help to understand the coupled heat and mass transfer.Thirdly,interactions between various components and the transported water molecule were calculated quantificationally and further compared.The concept of accumulated interactions was proposed to easily estimate the relative magnitudes of interactions caused by different components.Relationships between the relative magnitudes of interactions and the water orientations were also built up.Results show that the Li~+particles produce much larger attraction for the bulk water than the halide ions,which reflects the results of the pure liquid system in equilibrium.However,halide ions play more prominent roles during the initial absorption processes due to the fact that the probability of at least one OH bond towards the solution side is over 80%.As the absorption goes on,the interaction between the single Li~+and the absorbed water molecule is promoted because of diffusing to the liquid bulk and adjusting the orientation around ions.The relative magnitudes of interactions caused by different components are roughly consistent.The attractive interactions impede water release but promote water absorption.Novel methods may help to enhance mass transfer from the aspects of controlling the water orientations in air as well as changing the distribution of ions.Finally,by analogy with chemical reactions,the free energy surfaces of water release were obtained based on umbrella sampling.It is found that water release has a huge free energy barrier but water absorption nearly has no,which means the mass transfer coefficients during dehumidification are much higher than during regeneration.The elevation of solution temperature and the reduction of solution concentration could lower the free energy barrier of water release processes.Experiments were done using falling-film gas-liquid contactors.It is found that under the same mass transfer potential difference,the mass transfer coefficients for dehumidification are still much higher than for regeneration.The employment of free energy barrier during water transport succeeds in predicting the variation of mass transfer coefficients in experiments,which fills the theoretical vacancy and sets a theoretical foundation for developing uniform transfer coefficient correlations.